Processes for making aluminum by electrolysis of alumina dissolved in a molten salt bath are known in the prior art. Electrolytic processes in commercial use today for making aluminum involve passage of an electric current between a cathode and an anode in molten salt baths containing cryolite or other fluoride salts. The aluminum and sodium fluorides predominate, and lesser amounts of potassium, lithium, calcium, and magnesium fluorides may also be included.
In order to reduce the consumption of anode material in conventional electrolysis cells containing alumina-cryolite mixtures, Marincek U.S. Pat. Nos. 3,562,135 and 3,692,645 proposed a layer of oxygen-ion-conducting material in direct electrical contact with the anode. The oxygen-ion-conducting material is preferably zirconium oxide stabilized in a fluorite lattice by addition of calcium oxide, magnesium oxide, or yttrium oxide. However, stabilized zirconium oxide dissolves in molten cryolite and cells of Marincek's design are not in commercial use today.
In order to reduce the operating temperatures of aluminum electrolysis cells, prior art inventors have proposed molten salt baths containing various mixtures of fluoride and chloride salts. Two issued patents disclosing molten salt baths containing both fluoride and chloride salts are Lewis U.S. Pat. No. 2,915,442 and Wallace et al U.S. Pat. No. 2,915,443.
One important advantage of cells operated with mixed fluoride and chloride electrolytes is a lower bath density of about 1.74 g/cm.sup.3 compared with about 2.2 g/cm.sup.3 in cryolite baths. This lower density improves the stability of the bath-metal interface and allows an opportunity for a reduced interpolar distance, thereby improving cell productivity. Another advantage is reduced operating temperature, thereby enabling the use of materials that eliminate any need for a frozen layer of bath surrounding the cell top and sides. Reducing the cell temperature also reduces metal solubility in the bath, thereby improving current efficiency. Additionally, the chloride-fluoride bath electrical conductivity is higher than in conventional fluoride baths, effectively reducing resistive losses and improving current efficiency.
Reduced operating temperatures also lower sodium solubility, thereby minimizing distortion of the cathode blocks and improving overall dimensional stability of the interior lining. Reduced temperatures also extend cell life by reducing formation of aluminum carbide and its erosive effect on the cell block.
In spite of the advantages of mixed fluoride and chloride electrolytes, they are not used commercially. One concern about such electrolytes is the potential for producing chlorofluorocarbon compounds at the cell anode.
A principal objective of the present invention is to provide a process for production of metals by electrolysis in a molten salt bath, wherein the production of chlorofluorocarbon compounds as a by-product of the process is avoided by interposing a solid oxide ion conductor between the molten salt bath and the anode.
A further objective of the invention is to provide a process for production of metals by electrolysis in a molten salt bath, wherein the process can be retrofitted to existing electrolysis cells.
A related objective of the invention is to provide a novel electrolysis cell for carrying out the process of the invention.
One important advantage of our invention is that a mixed fluoride-chloride molten salt bath enables cell operation at a lower temperature than with all fluoride molten salt baths. Lower temperature operation reduces corrosion on the solid conductor of oxide ions, and enables operation at higher current densities without forming a crust on the cathode. Lower temperature and increased current density in a conventional, all fluoride molten salt bath precipitates sodium at the cathode thereby increasing voltage drop so that the cell eventually loses its ability to pass current.
Additional objectives and advantages of the present invention will become readily apparent to persons skilled in the art, from the following detailed description.